Now All Children's Hospital is taking the hardest line yet. Any employees who do not have a vaccination by 6 p.m. Friday may be fired unless they have an accepted medical or religious reason to decline.

"It's a patient safety issue," said Jay Kuhns, vice president of human resources at All Children's. "We take care of some of the most medically fragile patients out there, and if someone doesn't want to embrace our commitment to patient safety, they're not going to be able to work at All Children's anymore."

Studies show that when flu vaccination is optional, only about 50 to 70 percent of health care workers get the annual shot. Making it mandatory ups compliance to better than 90 percent.

The area's largest health system, BayCare, is giving workers at hospitals such as St. Joseph's, St. Anthony's and Morton Plant until Jan. 5 to either get a shot or wear a mask.

Tampa General and the HCA hospital chain already require most employees to get annual flu shots or wear masks.

But none is as strict about granting vaccine exemptions as All Children's.

What's behind the push?

Flu is most dangerous to the very young, the very old, and those who have other illnesses — in other words, hospital patients.

All Children's now is part of the Johns Hopkins Health system, which adopted a similar flu vaccine policy a year ago.

Plus, in the next few years hospitals will have to show that at least 90 percent of their employees have had a flu shot as part of the all-important accreditation review process.

In September, the traditional start of flu season, All Children's launched a system-wide drive to vaccinate everyone, including non-employee physicians, medical students in training, drug and medical device representatives and volunteers.

Everyone who receives an exemption must wear a surgical-style mask during flu season whenever they could come within 6 feet of a patient.

Dr. Juan Dumois, director of pediatric infectious disease at All Children's, acknowledged that there has been some grumbling about the policy. But he said that fewer than 20 exemption requests have been submitted; they are being reviewed by a committee at Johns Hopkins.

His research shows that today's objections to vaccines mirror historical concerns.

"Some of the same reasons have been around since the 1800s when the smallpox vaccine was introduced," he said. "Back then in England you could be jailed for refusing the vaccine."

As in the general population, some health care workers believe the flu vaccine could cause them illness, though the virus used in the serum is inactive. Some say they don't believe that it works — and researchers agree it's not 100 percent effective, though it's significantly better than going unvaccinated. Still others believe that they are naturally resistant to the flu — and it's true that a bout of flu may not be a big deal to a healthy person.

But for a vulnerable patient, "flu is potentially not a minor disease,'' Dumois said. "People die every year from the flu."

Most hospitals with vaccination requirements don't press employees who refuse shots — they just insist that they wear masks.

• Bayfront Health St. Petersburg (formerly Bayfront Medical Center) since 2005 has asked employees, volunteers, medical students and vendors to get a flu shot. Those who don't must wear a mask when in patient care areas inside the hospital.

• HCA West Florida, which has 13 hospitals in the area, including Brandon Regional Hospital, Largo Medical Center and Edward White Hospital, since 2009 has required employees, physicians, vendors and volunteers to get a flu shot or wear a mask when in patient care areas.

• Moffitt Cancer Center adopted a mask or vaccine policy in 2011. All unvaccinated workers must wear a mask when they are not in a private office or workstation during flu season. "So if they get up to go to the copier or go to the restroom, cafeteria, etc., they are required to wear the mask," said Michele Talka, Moffitt's director of Human Resource Operations.

• Last year, Tampa General Hospital implemented its vaccinate or mask policy, and sweetened the deal: Vaccinated employees who are on the TGH health insurance plan will save $130 next year on premiums.

• Employees and volunteers at BayCare hospitals have until Jan. 5 to comply. "They have two choices, either get the shot or wear a mask," said hospital spokeswoman, Beth Hardy. The unvaccinated must wear a mask inside any BayCare building.

• Florida Hospital requires anyone who isn't vaccinated to wear a mask if they will be within 6 feet of a patient.

Cynthia Sass, an employment law attorney in Tampa, explained that generally employers can require the vaccine as long as they are not required of people who can't take the shots because of a disability or religious reasons. In those cases, employers need to consider other options, such as masks. But employers must not do anything that would "harass or retaliate against the employee for failing to be vaccinated or has a stigmatizing impact on that employee," she said.

Contact Irene Maher at imaher@tampabay.com

.Fast facts

How bad is this year's flu?

As of last week, 38 Florida counties, including Pinellas, reported mild flu activity to the U.S. Centers for Disease Control. Hillsborough and Pasco counties had enough cases to edge into the moderate flu activity category. If you haven't had a flu shot, remember it takes a couple weeks after vaccination for immunity to develop, so don't delay.